Mr Khalilzad has urged the generals to embrace reforms
Burma's military rulers are no closer to accepting democratic reform, the US envoy to the United Nations has said.
Zalmay Khalilzad was responding to a report from the UN special envoy to Burma, Ibrahim Gambari, who said the generals were making concrete changes.
Burma's envoy to the UN, Kyaw Tint Swe, said it was "disappointing" that people "continue to express scepticism".
The row came before a major gem sale in Rangoon - a key source of revenue for the junta.
Human rights groups have called for a boycott of the two-week sale.
Mr Gambari had told the Security Council there had been "positive outcomes" from his latest trip to Burma - his second since troops violently suppressed anti-government protests in September.
The UN envoy - who was not allowed to meet top leader General Than Shwe - described the situation as "qualitatively different from what it was a few weeks ago".
"On balance, the positive outcomes of this latest mission show that the government of Myanmar [Burma]... can be responsive to the concerns of the international community," he said.
Ibrahim Gambari met opposition leader Aung San Suu Kyi last week
After his visit, detained pro-democracy leader Aung San Suu Kyi was allowed to issue a statement for the first time in over four years and to meet members of her party.
But Ms Suu Kyi remains under house arrest and Western diplomats, including Mr Khalilzad, have voiced scepticism over the commitment of Burma's leaders to a genuine process of dialogue.
"We do not believe that a fundamental shift has occurred in the regime's attitude to embrace substantive reconciliation and transition to democracy," the US envoy said.
In response Mr Kyaw, who represents the junta at the UN, said Burma's problems were "complex and delicate".
"It is disappointing that, notwithstanding the positive developments, some continue to express scepticism with regard to the commitment of the leadership and the pace of the ongoing process."
The BBC's Laura Trevelyan, at the UN, says Security Council diplomats are divided over whether the generals are genuinely committed to change, or just playing for time by pretending to engage with the UN and Aung San Suu Kyi.
Meanwhile in Rangoon, a major auction of precious stones is expected to attract hundreds of international dealers.
Hundreds of gem buyers are expected at the sale
Burma is one of the world's biggest producers of jade and gems such as rubies.
But human rights groups say they are mined using forced labour, and their export helps fund the junta.
"The sale of these gems gives Burma's military rulers quick cash to stay in power," Arvind Ganesan, of Human Rights Watch, said in a statement.
The group called for sanctions on Burma's gem trade - but most of the buyers expected to attend are from Asian nations such as Thailand and China which oppose sanctions against Burma.